Abstract The generation of novel transgenic rat models has greatly accelerated with the advent of nuclease-based genome-editing technologies such as ZFNs, TALENs, and CRISPR/Cas9 systems. With that in mind, there is a need for reliable methods of sperm cryopreservation using rat sperm, to ensure the future availability of these models. Existing methods are technically challenging, limiting their use by many research laboratories. Preliminary data demonstrate that supplementation of sperm freezing extender with the antioxidant monothioglycerol or the nonionic compound iodixanol enhances post-thaw motility. Proposed studies in Aim 1 will determine the effects of such supplementation on post-thaw motility, acrosomal and membrane integrity in sperm from multiple rat strains. Additionally, the effect of supplementation on fertility using cryopreserved sperm will be assessed using artificial insemination and in vitro fertilization. The data generated in this aim will serve to optimize rat sperm cryopreservation, thus enhancing the resources provided by the RRRC and the use of rat models in general in biomedical research. There is also increasing recognition that the gut microbiota (GM) plays a vital role in host physiology, metabolism, disease susceptibility, and behavior. As such, differences in the composition of the GM due to any number of factors may affect study reproducibility. Preliminary data show that the GM differs substantially between different genetic backgrounds of mice, even when purchased from the same commercial source. Of note, those differences are associated with differences in the phenotype of a well-characterized rat model of colorectal cancer. The studies proposed in Aim 2 will determine the influence of several common variables associated with animal husbandry, as well as genetic background and commercial source of animal, on the composition of the GM. Additional studies will examine the efficacy of various means of manipulating the GM toward a desired composition. Such studies hold the potential to greatly refine many rat models and are expected to generate broadly applicable data.